The prior art recognizes a number of methods for the revaporization of liquefied natural gas with attendant energy savings. Revaporization of liquefied natural gas by means of recycling a condensing medium in heat exchange with the natural gas is disclosed in U.S. Pat. No. 3,479,832. That patent utilizes a single circuit of a multicomponent heat exchange medium which is exchanged with the vaporizing natural gas.
Recovery of power during the vaporization of liquefied natural gas by a single expansion of a condensable circulating refrigerant, such as ethane or propane, is disclosed in U.S. Pat. No. 2,975,607. In addition, the latter patent discloses the use of sea water to provide an ambient heat source for the refrigerant. An improvement of this cycle is described in the paper entitled "Power Generation From Cryogenic Machinery," presented at the LNG-6 Conference held in Tokyo, Japan from Apr. 7-10, 1980 and authored by Shigeetsu Miyahara. The improvement involved reducing the number of modules in the main heat exchanger while still relying on a single expander for power recovery.
U.S. Pat. Nos. 3,293,850 and 3,992,891 disclose power recovery processes employing noncondensing gaseous heat exchange fluids during vaporization of the liquefied natural gas. Both patents require the use of fuel combustion to provide heat input to the exchanging systems. Cascade refrigeration systems for vaporizing liquefied natural gas streams, from which power is recovered by means of expanders, are shown in U.S. Pat. Nos. 3,068,659 and 3,183,666. Both patents disclose the need for heat sources, such as waste heat means or natural gas combustion.